# Why are alkali salts so soluble?

There's a certain solubility rule stating all carbonates, phosphates and hydroxides are insoluble unless mixed with hydroxides or alkali metals.

Why is this? What makes the alkali metals so special?

So the reason for carbonates and phosphates being insoluble in water because in an acidic environment, they are both protonated and lose their negative charges. The symmetry of their binding groups produces identical resonance structures at high pH values (since rotating one double bond will shape it into a superimposable structure of another). This allows any given oxygen at a time to posses the double bond (in reality the pi bond to the phosphorus or carbon is shared throughout all the bonded oxygens and each oxygen maintains an equal distance away from the central atom).

This partial negative charge (created by lack of firm placement of the pi electrons) can be neutralized by bonded hydrogens (this will create a small positive charge, however hydrogen's ionization energy is much less than oxygen's electron affinity). The lack of polar-ness of the phosphates and carbonates makes them insoluble in water.

In a basic environment, (created by hydroxides) the carbonates and phosphates will become deprotonated and charged. The charge on these molecules will allow them to be more miscible with water.

https://en.wikipedia.org/wiki/Carbonate

Since purely ionic bonding cannot exist, the percent ionic character of a bond is often useful when comparing the salts of different metals. For reference, an electronegativity difference of 1.7 is estimated to have about 50% ionic character, and 50% covalent character$^{[1]}$.

This being said, alkali metals comprise the least electronegative group in the entire periodic table (see here). This gives these metals some of the greatest electronegativity differences when forming bonds. The electron pair is so closely attracted to the electron acceptor that they are practically transferred to it.

The general trend is for electronegativities to decrease down the group, making lithium the least electronegative of the alkali metals. As such, some of the salts of lithium have different properties: $\ce{LiF}$ is not soluble in water, $\ce{LiI}$ is soluble in most organic solvents, and $\ce{LiOH}$ is the only non-deliquescent alkali hydroxide$^{[2]}$.

${[1]}$ Wikipedia, Ionic Bonding, Comparison with Covalent Bonding
$\ce{[2]}$ Wikipedia, Alkali Metal, Electronegativity